The present invention relates to an image processing device and an image processing method, particularly to an image processing device and an image processing method for identifying a pupil region.
Recently, there is developed a technology of automatically correcting a face image of a person captured by a digital camera or a camera-equipped mobile phone. In the face of the person, particularly an eye attracts attention of an observer of the image. As to correction performed to the eye, for example, it is conceivable to correct a red eye caused by a flash. There is also a technology of changing (improving) an appearance by enlarging the eye or a pupil in the face image. In the case that the eye or the pupil is corrected, a pupil region of the eye is accurately detected.
For example, a technology described in Patent Document 1(Japanese Unexamined Patent Publication No. 2009-181424) is well known as a method for detecting a position of the pupil. In a configuration described in Patent Document 1, a circular pupil is detected using a Hough transform. At this point, using a difference in density gradient between the eye captured under near-infrared illumination and the eye captured under visible-light illumination, a fan-like voting region is set to control generation of a pseudo voting value peak.
However, in the conventional configuration, unfortunately the pupil region cannot accurately be detected.
Usually, in the case that the circle is detected by the Hough transform, an edge (an outline) of luminance is extracted from an image as preprocessing, and the circle is detected by voting for the edge by the Hough transform. However, edges (such as upper-eyelid and lower-eyelid outlines, an eyelash, a wrinkle, make-up such as an eyeliner, and an edge by a shadow) except the pupil are detected as noise from an image of a surrounding region including the eye used in image processing. Particularly, the edges of the upper eyelid and the lower eyelid are long, and the edges extend over many pixels. Because the edges of the upper eyelid and the lower eyelid are frequently close to a circular shape or an elliptic shape, the number of votes for the circle fitting to the upper eyelid or the lower eyelid increases, and there is a possibility of falsely detecting the position of the circle indicating the upper eyelid or the lower eyelid as the position of the pupil.
In the face image, the entire pupil outline is rarely seen. Particularly, in the case that a person has narrowed eyes, frequently the upper and lower outlines of the pupil are hidden behind the upper eyelid and the lower eyelid. In the conventional configuration, because the number of votes by the edge of the pupil decreases, the position of the circle indicating the upper eyelid or the lower eyelid is easily falsely detected as the position of the pupil.
As described above, frequently part of the pupil region is hidden behind the upper eyelid and the lower eyelid. Therefore, in order to accurately detect the pupil region from the face image, not only the pupil outline as the circle is detected, but also boundary lines (boundary lines between the eye and a skin) of the upper eyelid and the lower eyelid are accurately detected. However, in the conventional configuration, the shapes of the upper eyelid and the lower eyelid are not considered, and the pupil region cannot accurately be detected.
Conventionally, the eyelid outline (the eye outline) is mainly approximated by quadratic curves (conic curves), such as the ellipse and a parabola. However, sometimes the eyelid outline cannot accurately be expressed by the quadratic curves. For example, the shape of the eyelid outline depends largely on individual differences, and frequently the eyelid outline cannot be expressed by the ellipse or the parabola. The eyelid outline becomes the shape that cannot accurately be expressed by the ellipse or the parabola depending on an orientation of the face captured in the image, particularly in the case that the face is not oriented toward the front. Unless the eyelid outline can accurately be detected, the pupil region cannot be identified with high accuracy.
The present invention has been devised to solve the problems described above, and an object thereof is to construct an image processing device that can accurately detect the pupil region.